Echo repeater and/or target simulator



Jan. 7, 1969 H. L, RATHBUN, JR 3,421,137

ECHO REPEATER AND/OR TARGET SIMULATOR Filed Feb. 2s. 1966 Sheet str2 firoe/ve Ys Ham/sy L. /P/m/u/v, Je. v

H. RATHBUN, JR

ECHO REPEATER AND/OR TARGET SIMULATOR Filed Feb. 234 196e Jan. 7, 19.69

Sheet INVENTOR. /M/e uw L, Ran/ um, Je.

United States Patent O Claims ABSTRACT OF THE DISCLOSURE An underwateracoustic repeater array wherein three equispaced transducers aresupported concentrically between the apexes of a pair of truncatedequilateral metallie pyramids. The outer surfaces of the pyramids arecoated with an acoustic reflecting material and the active faces of thetransducers are directed toward the coated faces. A housing having anamplifier therein connected :between two such arrays and includingelectrical means connecting in parallel one set of the transducers tothe amplifier input and the output thereof to the other threetransducers. The acoustic signal received #by one set of transducerswill be amplified and re-radiated by the other transducer set at ahigher level with a minimum of coupling therebetween.

The invention described herein may be manufactured and used by or forthe Government of the United States 'of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to acoustic arrays and more particularly to animproved acoustic echo repeater for underwater communication andretransmission omnidirectionally in a horizontal plane.

In general a typical underwater acoustic repeater array consists of ahydrophone array for receiving and a projector array for transmission.The term array for purposes of this specification is defined as one ormore transducers arranged in a particular geometry. Inherent in allrepeaters is the fact that a certain proportion or level of signal isfed back from the projector to the hydrophone. A basic prerequisite ofany such repeater is that for a given acoustic gain the level of thesignal fed back from the projector to the hydrophone through theacoustic medium and the mounting and support structure must be less thanthat of the original signal received by the hydrophone.

Solutions to the problem of feedback in acoustic repeaters have involvedthe yuse of rather large transducers in terms of wavelength and largebaies between the hydrophone and projector. Further, the separationtherebetween is made large so as to produce a bulky, weighty andunwieldy structure. The separation distance is directly limited by theFresnel diffraction around the array baille. Typically, for presentlyavailable equipment operating at approximately 12 kc. (}\(wavelength)=5inches) with a 25 db acoustic gain, the transducers are separated adistance of l2 feet (28.23A) and two circular aluminum air iilled rubberbatiies (diameter (7.2 \)=3 feet) are disposed between the projector andhydrophone.

Where the above dimensions are impractical or Where the wavelength islong, the only other solution has been to hold the receiving hydrophoneinoperative during transmission by the projector. Another solutioninvolves the use of transponders where a fixed, permanent signal levelis projected independent of the received signal characteristics. Thedrawback here is quite obvious since the projected signal is independentof the received signal, whereas the repeater should provide only a fixedgain or level in reference to the received signal.

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In view of the foregoing, it is an object of this invention to providean inexpensive, simple and reliable improved acoustic echo repeaterhaving a xed gain and of a minimum size and weight.

Another object is to provide azimuth oriented omnidirection acousticrepeater or target simulator having exteremely low coupling 'between thehydrophone and projector elements.

A further object is to provide an acoustic repeater array wherein theseparation between the active elements is minimized and the structuresthereof are of simple geometric shapes and proportions.

Other objects and advantages will appear from the following descriptionof an example of the invention, and the novel features will beparticularly pointed out in the appended claims.

In the accompanying drawings:

FIG. 1 shows a transducer suitable for use in the embodiment of theinvention;

FIG. 2 shows a perspective view of the bilaminar electrostrictiveelements with their support structure and electrical connection;

FIG. 3 illustrates in -plan an embodiment made in accordance with theprinciple of the invention;

FIG. 4 is a view taken approximately along 4-4 of FIG. 3;

FIG. 5 is a side view taken approximately along line 5-5 of FIG. 4, and

FIG. 6 is a graph illustrating the relative feedback between elementsfor various separations therebetween.

In the transducer of FIG. 1, bilaminar electrostrictive elements 10manifest dishing distortion in response to a driving alternatingpotential. Methods and materials for fabricating the laminae 11 and 12for electrostrictive elements 10 are well known in the art. For example,U.S. Patent #2,486,560 describes electrostrictive transducers,particularly of barium titanate and methods of making the same, whichdescription may be utilized for fabricating the transducer elements.Also, a paper published by Sperry Gyroscope Co. of Great Neck, N.Y., andpresented at the 14th Annual National Electronics Conference, Chicago,Ill., Oct. 13, 1958, entitled The Electro-Acoustic Transducer and ItsApplication to Sonar Systems, by George Rand and John Devine, includes`further information on electrostrictive transducers and methods ofmaking them. The laminae 11 and 12 are conventionally fabricated of amaterial that is or that can be rendered electrostrictive and theiropposed faces are coated with separate electrode films and are polarizedtransverse to the electrode tilms as is well known in the art, and thepolarization may -be carried out as described in the above-mentionedreferences. Paired laminae 11 and 12 are bonded face-to-face with anadhesive. The particular adhesive is not critical but the followingphysical properties in the adhesive to some degree determine operationalcharacteristics 0f the transducer. The more firmly that the adhesivebonds to the facing electrode surfaces of the electrostrictive element10 and the tougher the adhesive, the greater the power handling capacityof the resultant transducer without rupturing at the adhesive bond. Thegreater the flexibility of the adhesive bond and the thinner theadhesive bond, the greater the efiiciency of the transducer because lesspower is lost in driving the adhesive bond material. One example of acommercial adhesive that has satisfactory properties for the purposedescribed is Eastman 910 cement. There is considerable literature onadhesives from which information on other satisfactory adhesives may beobtained. For example, a book entitled Adhesives by Felix Braude,published by Chemical Publishing Company, and a periodical entitledAdhesives and Resins, published in Great Britain at 329 Grays Inn Road,London, W.C. 1, provides information on adhesives and their propertiesfrom which information on other adhesives satisfactory for the purposemay be selected.

The arrangement shown in FIG. 1 wherein one terminal is connected toboth facing electrodes is simple to assemble. One bonding procedure thathas proved satisfactory is to select a matched pair of electrodesurfacedand polarized laminae, apply adhesive to one face of each of the matchedlaminae, and with a thin ilat strip of copper foil disposed between theadhesive coated faces of the laminae, press the laminae firmly together.By applying pressure not only is a good adhesive .bond obtained, but thecopper foil 15 is forced into electrical contact with the facing filmelectrodes of the two laminae to a suflicient extent satisfactory forthe purpose. lConductors are conventionally soldered to the outside filmelectrodes and are connected in common to provide one electricalterminal of the transducer, and the foil 15 or a conductor connected tothe foil 15 provides the other electrical terminal of the transducer.

The two transducer elements are attached together in line. A spacer 16that gives satisfactory results is a ring formed with slots on its innerand outer surfaces at equiangularly spaced intervals and consecutiveangularly spaced slots occurring alternately on the inner and outersurfaces. The ring 16 may be formed from a stiff resilient material,e.g., brass tube stock, eg., SAE 74. To forni the ring, a length of thetubing stock is mounted in a band saw with an indexing means and itsouter surface is formed with slots. To form the inside slots, thecutting saw band is severed, threaded through the tubing, and its endswelded together and with the aid of indexing means the inner surface ofthe tubing is formed with slots. The wall thickness of the tubing isabout s inch and slot depth is on the order of 176,2 inch. Successiveslots may be on the order of ten degrees apart, the slot spacing isrelated to the circumferential length of the ring. For a 11/2 inch ringdiameter, ten degree slot spacing is suitable. After the tube stock isslotted, the tubing is sawed into thin rings. The ring thickness may beon the order of 1/16 inch. Two transducer elements 10 are so arranged onopposite sides of the ring for flexure in opposite directions when analternating potential is applied and the ring is bonded to the marginalareas of the inner faces of both transducer elements 10; an air space issealed in between the transducer elements 10. The spacing ring isradially compliant to dynamic forces but is radially stiff to staticforces, and it has high dynamic and static stiffness in the axialdirection. The radially compliant support afforded by the slotted ringendows the double bilaminar disk with excellent electromechanicaltransducing properties. The strain and the forces developed in one diskcorrespond to that in the other disk and with a radially compliant ringtherebetween, efficiency is high. If the metal ring were not radiallycompliant, i.e., if the ring were not slotted, it would prevent radialmotion of the disk edges when excited by an applied alternatingpotential and would thereby inhibit or even prevent bending or dishingaction. Because the ring has high dynamic stiffness in the axialdirection, each disk has, at its bonded margin, a node of axial motion.If the ring material were very compliant axially, for example, if itwere of rubber to provide good radial compliance, then the nodal circleof axial motion for each disk would move inwardly from the edge of thedisk, and the portion of the disk outside of this circle would vibrateout of phase, resulting in poor radiation loading.

With the above arrangement there is obtained an electroacoustictransducer with high electromechanical coupling coeflicient and withhigh strength.

When an alternating potential is applied to the double bilaminartransducer to drive the elements 10 in the fundamental exural mode, eachelement 10 manifests a dish-like distortion. The electrical connectionsto the electrodes and the directions of polarization are such thatdistortion in the two bilaminar disks are degrees 0ut of phase. Bydriving two bilaminar elements back to back as edge supported disks, thetransducer radiates from both outer faces. Substantially no energy isconsumed by the included air space. This unit when placed in an acousticmedium such as water will couple most of the acoustic energy into thewater with little loss in the included air cavity pressure release. Thisarrangement has the advantages of transducer elements anchored withheavy mounting fixtures without the disadvantages.

The mechanical resonance of the described transducer is determined bythe physical dimensions of the disks. The first resonant frequency inair of the mode of an edge supported diaphragm is dened as follows:

Berra-a f=resonant frequency, c.p.s. t--thickness R=radius cp=velocityof sound in the material E=modulus of elasticity p=density of thematerial r=Poissons ratio A transducer in accordance with this inventiondesigned for resonance at about 9 kc. is about 11/2 inches outsidediameter and slightly more than i716 inch thick overall (inside diameter15/16 inches). Because of small size and light weight, these transducerscan be assembled in arrays or mounted in appropriate reflective baies ashereinafter described to produce desired beam width and power handlingcapabilities. The transducing material is used to best advantage;substantially all of it is effective. Because the two opposed faces ofthe transducer radiate acoustic energy when alternating potential isapplied, the radiation loading that is obtained is greater than for adisk radiating from only one surface resulting in higher electroacousticefficiency. A transducer provides a pattern which is approximatelyomnidirectional and which approaches that of a spherical sound source.

The bilaminar piezoelectric ceramic transducers 20, 18, 19 describedhereinbefore are arranged coplanar and at the vertices of an equilateraltriangle as shown in FIG. 2. The transducers are supported by acentrally situated support member 21 via the outer brass rings or tubing22 although other support means and/or structure could be equally wellemployed. The active transducer elements are cast in an acousticallytransparent resin (generally indicated at 23) and electrically joined inparallel via cables 24. The support member 21 which may be a heavywalled brass tube also supports a pair of oppositely extending truncatedtriangular equilateral pyramids 25 as shown in FIG. 3. These pyramidsserve as reflectors or baliles 26 and are generally of some suitablemetal, such as aluminum coated with a reflective rubber material 27. Thepyramid walls 25 form 90 corner reflectors with the transducers disposedin the bisecting angle thereof. The layer of conventional compressionalwave reflective material 27 is bonded to the inner surfaces of the walls26 which are the outer pyramid surfaces. Two properties of the materialselected should be greatly different from the corresponding propertiesof the medium in which the array is used, namely, density and thevelocity of the compressional wave energy therethrough. Isoper, aproduct of B. F. Goodrich Industrial Product Company, Akron, Ohio, whichis a relatively stiff rubber-like material, is one example of a suitablecommercial material for layer 26 where the array is intended for use inwater.

The two pyramid structures are joined at their apexes by the supportmember and the transducers centrally disposed thereof with their activefaces directed toward the coated pyramid surfaces so as to form one-halfof a repeater or echo array. An identical unit is joined in axialalignment therewith by a shaft 28 and an electronic section 29'. Thetransducer elements are connected to the electronic section via cable 29as shown in FIG. 2. The dimensions shown in FIGS. 3, 4, and 5 are all interms of wavelengths and in this regard for use at approximately 9.0 kc.one-half inch thick Isoper cemented onto 3%6" thick sheet aluminum wasfound to provide satisfactory results. A center support shaft 30 whichcan, as illustrated, extend outwardly from the array and support thearray in the water.

Measurements were made to ascertain the effect of separation(s) betweenthe pyramids or baffles on the feedback loss. The results were plottedand are graphically illustrated in FIG. 6. This shows that a slightincrease in attenuation is achieved by separating the baflies but thatoverall a minimum of 30 db obtained even yfor small baie separations. Ingeneral the array is employed as an echo repeater and/or targetsimulator so that one Set of transducers are used as hydrophones whilethe other set acts as the projectors. The system is omnidirectional inthe plane of the transducers and since the units are identical they maybe interchanged. With a self-contained electronic system 29 the signalsreceived by one set of transducers (hydrophones) are `amplified andapplied to the projector set for retransmission at higher power withoutappreciable feedback.

summarizing the overall system, a clear prime requisite of a repeaterarray is that for a given acoustic gain the level of the signal fed backfrom the projector to the hydrophone, through the acoustic medium andother structure must be less than that of the original signal receivedby the hydrophone. The array essentially comprises a pair of identicalunits, the total, made up of four truncated equilateral pyramids havingbase lengths approximately 2\/ 3 wavelengths, two transducer units and awatertight housing containing sutiicient electronics, etc. The truncatedpyramids are lined or coated on the outside with an acoustic reflectivematerial. Bilaminar piezoelectric ceramic discs, edge-supportedback-to-back, operating in the fundamental flexural mode are employedsince they are both small and eflicient both as projectors andhydrophones. The transducer units consist of three coplaner discslocated at the vertices of an equilateral triangle whose side lengthshould be slightly less than one-half a wavelength. The center mount ofthe transducer unit is at the center of the triangle and is keyed so asto perform two functions, positioning the unit elements (discs) in thebaflie and accurately positioning the two bafe elements which whenassembled so that one pyramid is inverted with respect to the other,form three 90 corner retiectors with apertures of approximately twowavelengths (27x). The discs are therefore positioned in the bisectingplane of the 90 corner reflector. It should -be noted that since the4distance between a disc and its opposite baffle edge is not constant(varying from V5 to \/5), the length of the ray paths also vary and thusthe effect of any Fresnel diffraction is reduced.

The units are mounted one above the other or in axial alignment with theelectronic housing therebetween. If the array is used with the axis inthe vertical then each unit or the array as a whole, isomn1d1rect1onal1n azlmuth.

With this array structure the system provides:

A nearly omnidirectional pattern in the horizontal plane,

A pattern 30-50 wide between the half-power points in the verticalplane,

A feedback level between the projector and hydrophone units at least 30db down,

An operating source level of at least db/ /microbar at one yard,

Minimum component and array size dependent on the wavelength.

It will be understood that various changes in the details, materials,and arrangements of parts (and steps) which have been herein describedand illustrated in order to explain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

I claim:

1. An acoustic repeater array having first and second units, whereineach of said units comprises:

a pair of truncated equilateral metallic pyramids having the outersurfaces thereof coated with an acoustic reliecting material,

three acoustic transducers,

ring support means holding said transducers at the vertices of anequilateral triangle concentric therewith,

said support means disposed between said pyramids and supporting saidpyramids with their apexes adjacent one another and in axial alignmentwhereby said transducers are positioned symmetrically therebetween andhaving their active faces directed toward said coated outer surfaces,

a housing,

amplifier means confined in said housing and having input and outputterminals,

electrical means connecting in parallel said three transducers of saidfirst unit to said input terminal of said amplifier means and connectingthe output terminal thereof to the three transducers of said secondunit,

whereby the acoustic signals received by said first unit will beamplified and re-radiated at a higher level by said second units.

2. The acoustic array according to claim 1 further including:

structural means for supporting said housing between said units andmaintaining said units in axial alignment.

3. The acoustic array according to claim 2 wherein said transducers arebilaminar piezoelectric ceramics.

`4. The acoustic array according to claim 3- wherein the height of saidpyramids is approximately equal to the wevelength of said signal and thedistance between the bases of said pyramids of a unit is 2h.

References Cited UNITED STATES PATENTS 3,031,644 4/1962 Hisserich et al.340-3 3,054,084 9/1962` Parssinen et al. 340-8 3,117,318 1/1964 Jones343-18 3,243,768 3/1966 Roshon et al 340-10 RICHARD A. FARLEY, PrimaryExaminer.

U.S. Cl. X.R. 340-5, 8

